A finger touch switch is an important input device for various types of electronic equipment, including but not limited to computer keyboards, microwave ovens, copier machines, electronic card readers, ATM machines, and the like. Basic sensing mechanisms for identifying the pressed (depressed) point or location typically include metal to metal contact, capacitive or conductive variations of transparent electrodes, magnetic field detection, optical intercept, piezoelectric strain effect, and so on. In order to detect small deformations at the pressed points, one or more thin layers or membranes, or flexible insulators are often utilized. However, such structures often experience wear or fatigue problems, and/or are vulnerable to vandalism.
Examples of such finger touch switch devices known in the art include reflective mode ultrasonic touch switch devices as described in U.S. Pat. No. 5,673,041, assigned to Measurement Specialties, Inc., and incorporated herein by reference in its entirety. Such devices are based on resonance of a substrate (typically composed of low loss material such as glass or metal), and a piezoelectric element at the back surface of the substrate which detects the resonance. The resonance forms very sharp peaks at integer multiples of half wavelength thickness conditions of the substrate. This resonance is damped by contact at the substrate's front surface by a finger which is detected by the piezoelectric element.
Another type of touch switch known in the art (piezo touch switch) is illustrated in U.S. Pat. Nos. 4,896,069 and 6,064,141. This type of touch switch is based on a different mechanism than the ultrasonic touch switch described above. In this case, bending strain of a housing plate (sensor plate) by a pressure force is detected by a piezoelectric element bonded at the back of the plate and used to control switching of electric appliances. Disadvantages of such piezoelectric touch switches include their lack of sensitivity for steady contact force or very low sensitivity for slowly increasing or decreasing force, as well as their undesirably high sensitivity to external vibration or mechanical shock.
On the other hand, reflective mode ultrasonic touch switches or sensors have little or no sensitivity to external vibration or mechanical shock. Further, ultrasonic touch sensors exhibit sensitivity to steady contact, and when contact force increases beyond a given point or threshold, the switch starts to work (i.e. is activated). However, during operation of ultrasonic touch switches, particularly in applications such as for use in kitchen appliances, surface contamination (whereby a contaminant such as butter, ketchup, oils, greases, or even pure water etc. on the finger adheres to the sensing area) has the same effect as a steady touch of a finger, and can thereby yield a false signal.
Other disadvantages of the ultrasonic reflective mode touch switch includes its less than desirable (i.e. low) sensitivity to a gloved finger or soft cloth, while for a piezoelectric touch switch, this type of device exhibits essentially the same sensitivity to pressure force, with its sensitivity essentially independent of the material which covers the finger.
Further shortcomings associated with conventional sensors (ultrasonic or bending strain type) include their difficulty in providing, or the complete absence of, self diagnosis functions.
Alternative systems and methods are desired.